docs: the documentation, starter templates and tutorials should use the .mdx extension

[slorber]

As part of the MDX 2 migration (https://github.com/facebook/docusaurus/pull/8288) we want to encourage the usage of the .mdx extension, as it's historically not good to use .md and compile with MDX

For example, tooling (IDE, Prettier...) doesn't understand JSX when inside .md files

Also related to:

• https://github.com/facebook/docusaurus/pull/8490
• https://github.com/facebook/docusaurus/pull/8507

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⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟢 90	🟢 97	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 72	🟢 100	🟢 100	🟢 100	🟢 90	Report

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Size Change: 0 B

Total Size: 889 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	71 kB
website/build/assets/css/styles.********.css	112 kB
website/build/assets/js/main.********.js	665 kB
website/build/index.html	40.9 kB

_{compressed-size-action}

[slorber]

Hmmm, actually not 100% sure we should always encourage using .mdx extension by default. Others (Nextra, Astro...) do not.

I will have to explore what works and what doesn't anymore when using the .md extension, it's not 100% clear but I think most things may keep working (admonitions, links, asset requires etc...)

[slorber]

Indeed most things keep working in commonmark, and we want to fix things that do not yet.

Related to https://github.com/facebook/docusaurus/issues/9092

Closing tis PR for now